©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at ABHANDLUNGEN DER GEOLOGISCHEN BUNDESANSTALT Abh. Geol. B.-A. ISSN 0016–7800 ISBN 3-85316-02-6 Band 54 S. 7–13 Wien, Dezember 1999 North Gondwana: Mid-Paleozoic Terranes, Stratigraphy and Biota Editors: R. Feist, J.A. Talent & A. Daurer Tectonic and Plate Tectonic Units at the North Gondwana Margin: Evidence from the Central European Variscides WOLFGANG FRANKE*) 2 Text-Figures Variscan Belt Central Europe Terranes Contents Zusammenfassung .......................................................................................................... 7 Abstract .................................................................................................................... 7 Text ......................................................................................................................... 8 References .................................................................................................................. 12 Tektonische und plattentektonische Einheiten am Nordrand von Gondwana: Hinweise aus mitteleuropäischen Varisziden Zusammenfassung Für die Zeitscheibe des Mitteldevons lassen sich im mitteleuropäischen Variszikum die folgenden plattentektonischen Einheiten unter- scheiden: ● Old Red-Kontinent, bestehend aus Laurentia + Baltica + Avalonia. Am Südrand von Avalonia waren akkretioniert: • ein silurischer Inselbogen (das Resultat der Schließung des Rheischen Ozeans), und • „gestrandete“ armorikanische Fragmente ● Schmaler Rhenohercynischer Ozean ● Saxothuringisches Terrane (oder Terranes), bestehend aus der späteren Mitteldeutschen Kristallinschwelle und dem saxothuringischen Becken ● Schmaler Saxothuringischer Ozean ● Bohemia (oder Perunica) ● Schmaler Moldanubischer Ozean ● Gondwana (oder aber ein weiteres Fragment des Gondwana-Nordrandes). Abstract In Middle Devonian time, the following plate-tectonic units can be recognized in the Variscides of Central Europe: ● Old Red Continent, consisting of Laurentia + Baltica + Avalonia, with •a Silurian arc (resulting from the closure of the Rheic ocean) and • stranded Armorican fragments accreted to its southern margin. ● Rhenohercynian narrow ocean ● Saxothuringian terrane or terrane(s) (basement of the Mid-German Crystalline Rise and basement of the Saxothuringian basin) ● Saxothuringian narrow ocean ● Bohemian terrane (or Perunica) ● Moldanubian narrow ocean ● Gondwana mainland (or another Gondwana-related terrane). *) Authors address: WOLFGANG FRANKE: Institut für Geowissenschaften, Justus-Liebig-Universität, Senckenbergstraße 3, D-35394 Giessen. e-mail: [email protected]. 7 ©Geol. Bundesanstalt, Wien; download unter www.geologie.ac.at Identification of plate tectonic units (continents, mi- sif, and its equivalents in the Harz Mts. along-strike to the croplates, “terranes“) and reconstruction of their migra- NE. These rocks represent a narrow Rhenohercynian tion and assembly is a complex task. It requires all kinds ocean (i.e., the drift stage of Devonian extension). Equi- of geological evidence, combined with palaeomagnetic, valents of the Giessen nappe and its basal Armorican biogeographic and palaeoclimate data. These latter slices are exposed in the Lizard and Carrick nappes of methods have large error bars, and mostly fail to reveal south Cornwall. minor features. Hence, any attempt to summarize the All these Rhenohercynian lithologies permit to deduce state of the art has to start from geological field evidence. the following plate tectonic development: subsequent to Therefore, the following abstract is “extended“, and has the formation of the Old Red Assembly (Avalonia + Baltica to be based upon the recognition of tectonostratigraphic + Laurentia), convergence between the Old Red and Ar- units in the Central European segment of the Variscan Belt morica produced an island arc, which was accreted to the (as they are seen today). These units are then translated, southern margin of the Old Red Continent. Continued with the aid of complementary data, into plate tectonic convergence added Armorican fragments. All these units entities. It is vital to distinguish between these two cate- were overstepped by Devonian shelf sediments. Further gories, which are often mixed up with each other. Major south, a narrow oceanic domain was developed. South- and minor tectonostratigraphic units (marked as bold ward subduction of the Rhenohercynian ocean com- and underlined respectively) are briefly described in order menced in the Frasnian, and was followed by underplat- from N to S; palaeogeographic interpretations are ing of pre-Devonian basement from the Late Tournaisian shown in bold italics. The locations of the tectonostrati- onwards. Parts of the oceanic domain were transported graphic units are shown in Text-Fig. 1; the plate kinematic as thrust sheets on to the northwestern foreland, carrying evolution is summarized in a cartoon (Text-Fig. 2). with them Devonian and pre-Devonian rocks of the distal The Rhenohercynian belt represents a passive plate passive margin (FRANKE & ONCKEN, 1995). margin, which originated from crustal extension in Early The Mid-German Crystalline High (MGCR) represents Devonian through to Early Carboniferous time. the source area for the synorogenic clastic sediments The basement of the Devonian sequences is compo- (active margin) deposited on the Rhenohercynian fore- site: Most of the extended continental crust belongs to land in Frasnian through to Late Carboniferous time. Pre- the Avalonian microplate, which forms the southern- sent day exposures in the Odenwald, Spessart and most element of an assembly known as the “Old Red Thuringian Forest are mainly composed of arc-related Continent”: the Old Red composite plate resulted from plutonic rocks of Late Devonian to Early Carboniferous the amalgamation of Avalonia with Baltica (in Silurian age (363–335 Ma). Magmatic arc rocks of Silurian age are time), and Avalonia/Baltica with Laurentia (accomplished exposed in the basement of the Permian Saar/Nahe basin in the Silurian/Early Devonian). This Avalonian basement W of the River Rhine, in the Spessart and in the Thuringian in the Rhenohercynian belt was formed or reactivated by a Forest. These are equivalents of the Silurian arc exposed Cadomian event between ca 600 and 540 Ma. Rocks of in the Phyllite Zone at the southern margin of the RH, but this age are exposed in the Ecker gneiss (a roof pendant in were tectonically underplated under the MGCR and later the late Carboniferous Brocken granite of the Harz Mts.) exhumed in the core of an antiform within the upper plate and in the Wartenstein gneiss at the S margin of the Rhe- (ONCKEN, 1997). For most of the metamorphic rocks con- nohercynian west of the River Rhine. There is no indica- tained in the MGCR, protolith ages and plate tectonic affi- tion of the latest Ordovician Saharan glaciation, and Or- liations are unknown, and relationships with areas further dovician volcanics in the British part of Avalonia (Wales) S are therefore uncertain. show intermediate palaeo-latitudes. The Saxo-Thuringian belt is a Cambro-Ordovician A more southerly part of the pre-Devonian basement is rift basin developed on Cadomian crust. Late Proterozoic exposed in the Saar/Nahe region west of the Rhine and in sediments with Cadomian deformation and low-grade the Taunus Mts. at the southern margin of the RH east of metamorphism are exposed in the core of the Schwarz- the Rhine (Phyllite Zone). These areas contain Silurian burg anticline and occur as pebbles within locally derived magmatic rocks (now in greenschist facies) whose geo- Frasnian conglomerates. Late Cadomian (550–540 Ma) chemical fingerprints indicate an active margin (arc) granitoids are documented in the Erzgebirge antiform and setting. This magmatic belt is taken to represent closure in the Lausitz (Lusatia, E of the Elbe Fault Zone). A second of the Rheic ocean between Avalonia and Armorica. pulse of rifting in the Frasnian is documented by bimodal Rocks of the same age occur as pebbles in the Carbonif- intra-plate volcanic rocks, and clastic sediments derived erous flysch of south Cornwall (DÖRR et al., 1999). from local halfgraben shoulders. The southernmost (i.e., the most distal) part of the Rhe- The basin was overthrust from the SE, by the active, nohercynian passive margin is locally preserved in tec- northwestern margin of the Tepla-Barrandian terrane (see tonic slices at the base of thrust sheets derived from the below). This is documented by tectonic klippen (Münch- south. These are Early Devonian debris flow deposits and berg, Wildenfels, Frankenberg) which contain (from top to turbidites with clasts of Ordovician quartzites and Silurian bottom) metamorphosed Tepla-Barrandian rocks, eclo- to Early Devonian limestones, all of which have faunas of gites derived from ca 500 Ma MOR-type mafic rocks, and “Bohemian” (i.e., Armorican) affinities. Ordovician accreted slope sediments of the Saxo-Thuringian margin. quartzites (?Armorica) are also present at the southern The par-autochthon is characterized by shelf and he- margin of the Rhenohercynian belt in the Harz Mts. mipelagic sediments. The continental crust of the par-au- The thrust sheets carrying these Armorican fragments tochthon represents a Saxo-Thuringian terrane. The are mainly composed of Early Devonian MOR-type me- continental source, which should be expected to the NW tabasalts, overlain by condensed deep-water pelites and of the shelf, has not been preserved: paradoxically, the synorogenic greywackes